The invention relates to a semiconductor device made of polymer that includes at least one rectifying function. In particular it relates to diodes and notably photovoltaic and electroluminescent diodes. It also relates to devices such as transistors.
The use of polymers to produce semiconductor devices is technically rather interesting. In effect, the polymers can be formed by a wet method from a solution, which leads to techniques which are easy to implement, that are low cost and which are compatible with other techniques.
In order to produce semiconductor devices such as electroluminescent diodes and photovoltaic cells, it is usually appropriate to create a junction. This can be a Schottky junction obtained by bringing a doped semiconductor into contact with a metal having a rectifier contact with the semiconductor used. It may also be a pn junction obtained by placing a p type semiconductor at the side of an n type semiconductor. The junction is called a homo-junction if the semiconductor material is the same for the whole of the junction. In the contrary case, one speaks of a hetero-junction.
It is known to use polymers to produce these two types of diode (Schottky and pn junction). As a function of the type of conductivity required, p or n, it is preferable to chemically dope the polymer with atoms (or molecules) that are respectively acceptors or donors of electrons.
A disadvantage of Schottky diodes produced by the juxtaposition of a metal and a polymer is their short life. This is due to the diffusion of the metal into the polymer where the two materials are in contact, because of the difference in the electrochemical potential that exists between these two materials.
Logically, a pn junction should allow this disadvantage to be remedied. It is produced by the juxtaposition of a type p polymer and a type n polymer. However, it is extremely difficult to produce such pn junctions since the choice of the different polymers is not always compatible with the operational imperatives. Furthermore, type n type polymers are not very common and are often unstable to oxygen.
Another problem inherent to polymer diodes of both of these types (Schottky and pn) is linked to the small breadth of the depletion layer at the junction which is less than 20 nm. So as to avoid short circuits between the electrodes, the devices made up of thin layers of polymer are produced with a thickness greater than or equal to 100 nm. As the mobility xcexc of the charges within the organic components is a function that increases rapidly with the internal electric field E of the semiconductor in accordance with the empirical law
xcexc=xcexcoxc2x7exp[(E/Eo)P]
where the exponent p is close to 0.5 and as the internal field E is only large within the depletion layer, the electrical charges do not pass well in polymer semiconductor devices of the known art. This leads to low efficiency for these devices.
This invention allows these disadvantages to be remedied. It consists of creating a rectifying homo-junction gradient equivalent to a pn junction in a single thin layer of polymer which is used as a host matrix for polar molecules orientated in an appropriate manner. The result is a significant increase in the mobility of the carriers within the entire thickness of the layer which leads to a uniformly distributed depletion layer. This principle in a significant way, improves the performance of the devices with a junction such as electroluminescent diodes, photocells and transistors produced from polymers.
Therefore a first subject of this invention consists of a semiconductor device made of polymer that includes at one rectifying function, characterized in that the function is produced by a layer of polymer between first and second means that form electrodes, the layer of polymer constituting a host matrix for polar molecules, the polar molecules being electrically orientated in a direction perpendicular to the first and second means forming the electrodes, the electrical charges of the polar molecules of same sign being directed towards the same means that form an electrode.
A second subject of this invention consists of a method of producing a polymer semiconductor device that includes at least one rectifying function, characterized in that it includes the following steps:
formation of a layer based on polymer and including polar molecules, the polymer constituting a host matrix for the polar molecules,
orientation of the polar molecules in the host matrix so that the electrical charges of the polar molecules of same sign are directed to the same side.